1. Field of the Invention
The invention relates generally to well drilling and completion fluids.
More specifically, the present invention relates to high density, thermally stable, brine-based well fluids.
2. Background Art
When drilling or completing wells in earth formations, various fluids typically are used in the well for a variety of reasons. The fluid often is water-based. For the purposes herein, such fluid will be referred to as xe2x80x9cwell fluid.xe2x80x9d Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of xe2x80x9ccuttingsxe2x80x9d (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, fluid used for implacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
One issue that arises in drilling operations relates to the thermal stability of the well fluids. Temperatures in subsurface formations generally rise approximately 0.5-1xc2x0 C. (gradient varies with location) per hundred feet of depth. Many well fluids contain additives that may undergo severe chemical degradation in the presence of elevated temperatures, which may lead to a variety of problems.
Brines (such as, for example, aqueous CaBr2) commonly are used as well fluids because of their wide density range and the fact that brines are typically substantially free of suspended solids. In addition, brines are often used in order to achieve a suitable density for use in well-drilling operations. Typically, the brines comprise halide salts of mono- or divalent cations, such as sodium, potassium, calcium, and zinc. Chloride-based brines of this type have been used in the petroleum industry for over 50 years; and bromide-based brines, for at least 25 years; but formate-based brines have only been widely used in the industry relatively recently (roughly the past ten years). One additional advantage of using brines is that brines typically do not damage certain types of downhole formations; and for formations that are found to interact adversely with one type of brine, often there is another type of brine available with which that formation will not interact adversely.
A variety of compounds are typically added to brine-based well fluids. For example, a brine-based well fluid may also include visosifiers, corrosion inhibitors, lubricants, pH control additives, surfactants, solvents, and/or weighting agents, among other additives. Some typical brine-based well fluid viscosifying additives include natural polymers and derivatives thereof such as xanthan gum and hydroxyethyl cellulose (HEC). In addition, a wide variety of polysaccharides and polysaccharide derivatives may be used, as is well known in the art.
Some synthetic polymer and oligomer additives such as poly(ethylene glycol)(PEG), poly(diallyl amine), poly(acrylamide), poly(aminomethylpropylsulfonate[AMPS]), poly(acrylonitrile), poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl amine), poly(vinyl sulfonate), poly(styryl sulfonate), poly(acrylate), poly(methyl acrylate), poly(methacrylate), poly(methyl methacrylate), poly(vinylpyrrolidone), poly(vinyl lactam), and co-, ter-, and quater-polymers of the following co-monomers: ethylene, butadiene, isoprene, styrene, divinylbenzene, divinyl amine, 1,4-pentadiene-3-one (divinyl ketone), 1,6-heptadiene-4-one (diallyl ketone), diallyl amine, ethylene glycol, acrylamide, AMPS, acrylonitrile, vinyl acetate, vinyl alcohol, vinyl amine, vinyl sulfonate, styryl sulfonate, acrylate, methyl acrylate, methacrylate, methyl methacrylate, vinylpyrrolidone, and vinyl lactam are also often used as viscosifiers.
One example of how a brine-based well fluid may be used in combination with the above listed polymers and oligomers is set forth below. When drilling progresses to the depth of penetrating a hydrocarbon bearing formation, special care may be required to maintain the stability of the wellbore. Examples of formations in which stability problems often arise include highly permeable and/or poorly consolidated formations. In these types of formations, a drilling technique known as xe2x80x9cunder-reamingxe2x80x9d may be used. In under-reaming, the wellbore is drilled to penetrate the hydrocarbon bearing zone using conventional techniques. A casing generally is set in the wellbore to a point just above the hydrocarbon bearing zone. The hydrocarbon bearing zone then may be re-drilled, for example, using an expandable under-reamer that increases the diameter of the already-drilled wellbore below the casing.
Under-reaming is usually performed using special xe2x80x9ccleanxe2x80x9d drilling fluids. Typical drilling fluids used in under-reaming are expensive, aqueous, dense brines that are viscosified with a gelling and/or cross-linked polymer to aid in the removal of formation cuttings. The high permeability of the target formation, however, may allow large quantities of the drilling fluid to be lost into the formation. Once the drilling fluid is lost into the formation, it becomes difficult to remove. Calcium and zinc bromide brines can form highly stable, acid insoluble compounds when reacted with the formation or substances contained therein. This reaction may reduce the permeability of the formation to any subsequent out-flow of targeted hydrocarbons. One of the most effective ways to prevent such damage to the formation is to limit fluid loss into the formation.
Thus, providing effective fluid loss control is highly desirable to prevent damaging the formation in, for example, completion, drilling, drill-in, displacement, hydraulic fracturing, work-over, packer fluid implacement or maintenance, well treating, or testing operations. Techniques that have been developed to control fluid loss include the use of fluid loss xe2x80x9cpills.xe2x80x9d Significant research has been directed to determining suitable materials for the fluid loss pills, as well as controlling and improving the properties of the fluid loss pills. Typically, fluid loss pills work by enhancing filter-cake buildup on the face of the formation to inhibit fluid flow from the wellbore into the formation.
Because of the high temperatures, high shear (caused by the pumping and placement), high pressures, and low pH to which well fluids are often exposed (i.e., xe2x80x9cstress conditionsxe2x80x9d), the above described polymeric additives used to form fluid loss pills, and to viscosify the well fluids, tend to degrade rather quickly. In addition, even in well fluids without these polymeric additives, other chemical species present in the well fluid may decompose or undergo undesirable reactions. Brine-based well fluids, therefore, are used for a variety of applications in well drilling, both with and without polymeric additives. Increasing the thermal stability of brine-based well fluids, therefore, is a significant concern.
U.S. Pat. No. 4,900,457 (the ""457 patent) describes one method of raising the thermal stability of these well fluids. In particular, the ""457 patent discloses an aqueous polysaccharide composition comprising 0.03 to 5% weight per volume (w/v) of a water-soluble polysaccharide, 5 to 120% w/v of at least one salt of at least one mono- or divalent cation, wherein at least 0.05% w/v, based on the composition of the at least one salt is formate, the balance of the salt, if any, being at least one halide. Further, the ""457 patent states that the most preferable amount of formate is 10% w/v.
High density, brine-based well fluids have become increasingly important for a variety of applications, such as well completion. Along with having high densities, these brine-based well fluids must also be thermally stable. What is needed, therefore, are high density, thermally stable, brine-based well fluids for use in a variety of applications.
In one aspect, the present invention relates to a high density, thermally stable well fluid, wherein the well fluid includes water and a salt, wherein the salt includes at least one formate salt, and wherein a weight-to-volume ratio of the salt in the well fluid is within the range of at least 120% to about 400%.
In another aspect, the present invention relates to a high density, thermally stable well fluid, wherein the well fluid includes water, a viscosifying agent, and a salt, the salt including at least one formate salt, wherein the weight-to-volume ratio of the salt in the well fluid is within the range of at least 120% to about 400%.
In another aspect, the present invention relates to a method of treating a well which includes injecting a well treating fluid into the well, wherein the well treating fluid includes water and a salt, the alt including at least one formate salt, wherein the weight-to-volume ratio of the salt contained in the well fluid is within the range of at least 120% to about 400%.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.